1. Field of the Invention
The present invention relates to an illumination optical system for guiding the light emitted from a light source to a light valve for forming an image, and a projection-type display apparatus having such an illumination optical system.
2. Description of Related Art
In the projection-type display apparatus, the illumination optical system thereof having capability of condensing and transmitting incoming light converts the light emitted from the light source into a light beam having uniform luminance distribution within its cross section with which the light valve thereof is illuminated. The light source may be a very-high pressure mercury lamp. The divergent light from the lamp is converted into collimated or convergent light by a concave mirror disposed near the lamp, and enters the illumination optical system. The light emitted from the lamp has, when condensed, a rotational-symmetry luminance distribution in which the luminance increases toward the center portion within its cross section normal to the optical axis. Such a luminance distribution is not suitable for illuminating the light valve, and it is therefore converted into a light beam that has a rectangular cross section and is uniform in luminance within the rectangular cross section.
One of methods for obtaining such an illuminating light beam uses a columnar optical device as an optical integrator. In this method, the light from the lamp is condensed, and enters the columnar optical device (a glass prism or a hollow-body mirror) at one end thereof within which it reflects several times to be uniformalized and exits from the other end as a uniform illuminating light beam rectangular in cross section (for example, refer to Patent document 1).
Another method uses two lens arrays. In this method, images of contours of rectangular lenses of a first lens array are formed on the input side of a light valve by use of lenses of a second lens array. This method provides a uniform illuminating light beam rectangular in cross section by integrating light beams exiting from a number of lenses of the first lens array (for example, refer to Patent Document 2).
When the light valve is a liquid crystal device, unidirectionally polarized light is used as the illuminating light beam. Since the lamp as the light source emits polarized lights having different polarization directions, if a polarized light having a first polarization direction is to be used for illuminating the light valve, it is necessary to convert a polarized light having a second polarization direction perpendicular to the first polarization direction into the polarized light having the first polarization direction to improve utilization efficiency of the light.
Methods for performing such polarization conversion includes the one that utilizes the columnar optical device (for example, refer to Patent Document 3), the one that utilizes he lens arrays (for example, refer to Patent Document 4), and the one that uses a polarization-separation prism, a phase plate and a mirror (for example, refer to Patent Document 5).
For providing a smaller and less expensive projection-type display device, it is desirable that the number of the light valves used is small. An apparatus that can project a full color image by use of a single light valve is known (for example, refer to FIG. 2 of Non-patent Document 1). This apparatus has a color wheel for performing color-switching among three primary colors. However, the utilization efficiency of the light emitted from the lamp is low, since an image is formed by one of the three primary colors at a time, and the lights of the other two of the three primary colors are therefore discarded. On that account, it has been proposed to illuminate a single light valve with two or more primary colors at a time to improve utilization efficiency of the light (for example, refer to Patent Document 6, Patent Document 7, and FIG. 9 of Non-Patent Document 1).    Patent Document 1: Japanese Patent Application Laid-Open No.: 07-98479 (FIG. 2)    Patent Document 2: Japanese Patent Application Laid-Open No. 03-111806 (FIG. 3)    Patent Document 3: U.S. Patent Application Publication No.: US2001/0008470 A1 (FIG. 9)    Patent Document 4: Japanese Patent Application Laid-Open No.: 2000-284229 (FIG. 1)    Patent Document 5: Japanese Patent Application Laid-Open No.: 63-121821 (FIG. 1)    Patent Document 6: Japanese Patent Application Laid-Open No.: 04-316296 (FIG. 1)    Patent Document 7: U.S. Patent Application Publication No.: US2002/0135862 A1 (FIG. 6)    Non Patent Document 1: Serge Bierhuizen, Single Panel Color Sequential Projectors with Polarization Recovery, SID'02 Digest-55.5 (FIG. 2 and FIG. 9)
However, the above-described conventional illumination optical systems have problems described below. In the case of using a columnar optical device as a light integrator, the columnar optical device has to be long enough, otherwise a sufficiently uniform illuminating light beam cannot be obtained.
In the case of using two lens arrays as an optical integrator, each of the lens arrays must have a large cross-sectional area, and a long distance must be put between them. As a result, the display apparatus becomes large in size.
In the case of utilizing the columnar optical device for performing the polarization conversion, the columnar optical device must have an aperture formed in its light-entrance surface. This gives rise to a loss when the light emitted from the lamp enters the columnar optical device. In the case of disposing a strip-like conversion prism in the rear of the lens array for performing the polarization conversion, it is difficult to manufacture the display apparatus at low cost since the strip-like conversion prism has a complicated structure and is expensive. In the case of using a polarization-separation prism, a phase plate, and a mirror is used to perform the polarization conversion, the display increases in weight and becomes complicated in structure.
In the case of using a color wheel for displaying a full-color image by use of a single light valve, the utilization efficiency of the light is low. In the case of using a rotatable prism for displaying a full-color image by use of a single light valve, the display apparatus becomes large in size and becomes complicated in structure. In the case of using a BMF (Band Modulation Filter) switch for displaying a full-color image by use of a single light valve, it is difficult for the display apparatus to have sufficiently long lifespan.
In the case of using a color wheel having color segments arranged spirally for displaying a full-color image by use of a single light valve, a light loss is caused since a columnar optical device having an aperture at its light-entrance surface has to be used for recapturing the lights reflected from the color segments. In addition, there is a problem that a slight displacement of the light source causes a substantial reduction of brightness of a projected image.
Furthermore, the conventional illumination optical systems in which reflective optical devices such as a reflective light switch, a dichroic mirror, a reflective polarization-separation device, and a reflective light valve are set such that their entrance surfaces are at right angles with the optical axis have a problem that the lights reflected from such reflective optical devices travel in the opposite direction towards the lamp, and part of the lights that have returned to the lamp causes the temperature of a lamp electrode to rise thereby shortening the lifespan of the lamp.